Lol, you'd be VERY unhappy running windows XP on a 233 mhz pentium mmx. Realistically, if you have a lot of memory, you can probably get away with running XP decently well on a 500 mhz range processor, but that won't be very fast. Anything much less is unusable.
If you are running a 500 mhz processor, or even a bit more (up to 1 ghz imo), you're much, much better off running 2K, or even 98 on a very slow machine. XP is just too much of a resource hog. The eye candy alone will eat up most of your cpu cycles.
I have a P133 laptop with 72MB RAM that really cannot run windows passibly well. I am currently running debian linux on it with icewm as a window manager, and it's pretty usable. Certainly not fast, but it's faster than Windows XP on a 500 mhz PC.
Also, remember that a socket 754 Athlon 64 running at 800 mhz is a LOT faster than a pentium III or K6-2 running at that same speed. I don't think a K6-2 can even run that fast, but what I mean is that the A64 just does a lot more per clock cycle. You can't really compare cpu power using just clock speed except between processors of the same architecture (like a P4 2.8 and a P4 3.0). Then there's that stupid Athlon rating system of numbers they pull out of their collective asses that have no connection to anything in the real world at all whatsoever. I wish they'd just put the real clock speeds on those.